If you just want to mess around on a track day and are not serious about your times, then use up some of your old tires.

But, to run fast consistent lap tires you need to start with a new set of tires and wear them on your car. If you purchase another persons used tires you have no idea what type of abuse the tires underwent before you
got them. Also, you've no idea of the
heat cycles that tire has gone through or even if all 4 have the same number of heat cycles.